Imagers typically consist of an array of pixel cells containing photosensors. Each pixel cell produces a signal corresponding to the intensity of light impinging on its photosensor when an image is focused on the array by one or more lenses. These signals may be stored in a memory and displayed on a monitor, manipulated by software, printed to paper, or otherwise used to provide information about the image. The magnitude of the signal produced by each pixel is substantially proportional to the amount of light impinging on a respective photosensor.
Several kinds of imagers are generally known. Complementary metal-oxide-semiconductor (“CMOS”) imagers and charge coupled device (“CCD”) imagers are among the most common. CMOS imagers are discussed, for example, in U.S. Pat. Nos 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, all assigned to Micron Technology, Inc.
Images generated from CMOS or other imagers typically comprise thousands or even millions of picture elements called “pixels” arranged in rows and columns. One or more values, each usually comprising 8 or more bits, are typically associated with each pixel. In a grayscale image, just one value corresponding to brightness is associated with each pixel. In color images, three or four values are associated with each pixel, depending on the color space used by the imager or processing software. RGB and YUV are two common color spaces. In the RGB color space, a red value (R), a blue value (B), and a green value (G) are associated with each pixel. In the YUV color space, a brightness value (Y) and two chrominance values (U and V) are associated with each pixel.
Digital image processing can be used to enhance or correct errors in color images. Aliasing artifacts are one common error. Aliasing occurs when detail in a scene exceeds the sampling frequency of the imager, for example, when the lines of detail in an image exceed the number of rows of pixels in the pixel array of an imager. Aliasing can result in false color artifacts along edges of details in an image and especially along edges involving an abrupt black-to-white transition. In the YUV color space described above, false color artifacts may be exhibited as pixels whose chrominance (U and V) values are too high, causing the pixels to appear intensely colored when they should be more muted or even gray. The tendency of lenses to refract different wavelengths of light differently, is another common cause of false color artifacts.
Correcting the root causes of false color artifacts requires additional hardware components or substitution of higher-quality hardware components, for example a pixel array with more pixels or lenses that refract varying wavelengths of light more evenly. These solutions are often impractical, particularly in low-cost imagers. Therefore, a less expensive method for correcting false color artifacts, particularly one which does not require additional or higher-quality hardware, is desirable.